Niobium Wire Research Articles

Precipitation kinetics in Nb(Cb)-N alloys

Niobium wires were doped with 0.2 wt pct N and then directly aged at temperatures from 300° to 650°C for various lengths of time. Metallographic examination revealed a product phase at subgrain boundaries which exhibited linear growth for each reaction temperature. An activation energy of 5.6 kcal per mole for the process was obtained. Although the reaction was thought to be discontinuous precipitation, additional information from electron microscopy, internal friction, and hardness measurements indicated that both precipitation and a coarsening process were occurring simultaneously

The resistive critical field of niobium (columbium) wire

The transverse and longitudinal resistive critical fields of niobium (columbium) wires with drawing strains up to eleven were measured. The magnitude and anisotropy of the resistive critical field can both be much greater than the bulk Ginzburg-Landau parameters indicate, even if “internal surface” nucleation of superconductivity is assumed. It is suggested that these effects are due to superconductivity in the dislocation cell walls created by drawing or swaging. A simple model for this effect is offered, and various experiments which test this model are discussed.

A New Type of Surface Ionization Detector

The ionization coefficient of a potassium atom impinging on a hot Pt-8% W wire is independent of its energy below 3 eV (3850 m/sec) and dependent on it above 3 eV. This is due to the fact that a high energy atom can reflect from the wire without being ionized. By using a tungsten cylinder as an ionizer and a niobium wire in the middle of it as a collector, incident atoms which are not ionized can be prevented from leaving the cylinder without being detected because subsequent collisions with the wall will take place and result in ionization. By means of this type of standard detector, which has a multiplication factor of about 30, the reflection coefficient of a potassium atom incident on a Pt-8% W wire is determined as a function of its velocity. For a potassium atom with an energy of 23.6 eV (10 800 m/sec) this reflection coefficient is about 16.5%.

Low temperature specific heat of annealed high-purity niobium in magnetic fields

The specific heat of annealed niobium wires with a resistance ratio of 145 was measured at constant magnetic fields in the temperature range from 1 K to 10 K. Analysing accurately the normal state data below 3 K θ was found to be temperature dependent. The specific heat in the mixed state for H > H c1(0) contains a term, which is linear in T and field dependent. The transition at H c1( T), which is not free from irreversibility at higher fields, may be defined as the large anomaly in the specific heat. The transition at H c2 is second order and its temperature dependence was investigated from 2.6 K up to T c = 9.28 K. At T ≠ T c the observed H c2( T s), expressed in κ1 and h∗, is higher than the theoretical predictions of Maki, Eilenberger, Helfand and Werthamer. The jump in the specific heat Δ C( T s) was found to vary at low temperatures as T 3. By means of an Ehrenfest relation, information about the magnetization has been obtained from Δ C( T s), which may be compared with the measured magnetization and the predicted temperature dependence of the parameter κ 2. At T = T c both the Maki parameters converge to the Gl parameter κ, which is found to be 0.893. κ 0 for pure niobium, from which other microscopic characteristics are calculated, is found to be 0.83. Niobium is found to be an intrinsic type II superconductor with an electron-phonon coupling of intermediate strength.

Critical state equation for superconducting niobium wires

Assuming j c = α ( 1 − |B| H c2 ) , equations are easily obtained which accurately describe I c vs. H for Nb wires in a simple arrangement simulating infinite slab geometry and readily yield a measure of the pinning parameter α.

Student Laboratory Demonstration of Flux Quantization and the Josephson Effect in Superconductors

A simple apparatus is described which permits observation of magnetic flux quantization and the alternating current Josephson effect in superconductors. The apparatus, which has been used successfully by students in the Advanced Undergraduate Physics Laboratory at Berkeley, consists of point contacts between two niobium wires which are attached to a probe so that the contact pressure can be adjusted while the contacts are in a storage Dewar of liquid helium. The current-voltage characteristic of the point contact junction is displayed on an oscilloscope. Flux quantization causes the zero-voltage current carried by the junction to oscillate as a magnetic field is applied by moving a permanent magnet close to the storage vessel. The ac Josephson effect causes constant voltage steps to appear on the current-voltage characteristic when a small amount of power from a K-band (∼24-GHz) klystron is coupled to the points.

Frequency Conversion Using Weak Multiply Connected Josephson Junctions

A Josephson tunneling device essentially consisting of a lead-tin solder bead encircling a 5-mil niobium wire has been used to produce some frequency conversion effects. Thin spots in the oxide coating of the niobium wire give rise to two or more parallel weak-contact regions between the solder and the niobium. A typical voltage vs junction current plot displays an antisymmetric tunneling curve with zero voltage across the junction until a certain critical current Ijc is supplied. A field current IH flowing through the niobium wire produces a magnetic flux which links the multiply connected region(s) formed by the penetration depths of the solder and the niobium and the distance(s) between the parallel weak contacts. The critical current Ijc is a periodic function of the magnetic flux threading the multiply connected region(s), the modulation period being proportional to one flux quantum. When the device is biased with a junction current Ij in the interval Ijc≤Ij≤3Ijc, a modulated dc voltage appears across the junction as IH is varied. The period of the oscillations is independent of the junction bias Ij. The number of cycles ΔN produced for a given absolute change | ΔIH | defines the junction constant Kj of the device. Kj depends upon the geometry of the device and the penetration depths of the superconductors used in its construction. If a time varying IH is applied to the niobium wire an output frequency f0 can be obtained, given by the relation f0=Kj | dIH/dt |. For a triangular IH the device acts as a frequency multiplier giving f0=4KjIHm fin, where IHm is the current amplitude of the triangular waveform and fin its frequency. A sinusoidal IH gives rise to a frequency-modulated output: f0=2πKjIHm fin | cos2πfint |. Other input waveforms yield other frequency-modulated outputs.

Strong Proximity Effects in Cu: Superconductivity of a Fine Nb–Cu Composite

The magnetic behavior of composites of very fine (less than 1000 Å) niobium wires in copper matrices has been measured at temperatures below 10°K. Three Meissner effects were observed, two of which were apparently due to strong proximity effects in the copper. The temperature dependence of the magnetization curve and measurements of the real and imaginary components of zero-field ac susceptibility agree. The geometry and properties of the composite, which are known in some detail, are used together with current models for proximity effects to develop an adequate explanation of the data.

Präparation und Supraleitungseigenschaften von Niobnitrid sowie Niobnitrid mit Titan-, Zirkon- und Tantalzusatz

A description of preparation for samples of niobium nitride without and with admixtures of titanium, zirconium and tantalum by diffusion of nitrogen under pressure into niobium wires without and with admixtures of titanium, zirconium and tantalum respectively, is given. For samples of pure niobium nitride with different compositions the influence of the nitrogen content on the transition temperature to superconductivity has been investigated with the result of a transition temperature not exceeding 16.5 °K even for the stoichiometric composition NbN. This is in contrary to other authors. For diffusion wires of the systems (Nb-Ti)N x , (Nb-Zr)N x and (Nb-Ta)N x the critical temperature, lattice parameter and nitrogen content as a function of the admixture of titanium, zirconium and tantalum respectively, are given. For the first time the magnetisation curve of niobium nitride has been measured showing the behaviour of type II superconductors. Concerning the kinetics of formation of niobium nitride by diffusion the nitrogen content is increasing for constant nitrogen pressure with raising temperature of diffusion, but is decreasing in the high temperature region.

Low temperature specific heats of annealed high purity niobium in magnetic fields

Specific heats of a bundle of annealed niobium wires of high purity were measured in the superconducting, mixed and normal states. At the lowest temperatures the specific heat in the mixed state for H > H c1(0) contains a field dependent term linear in T. The observed temperature dependence of the Maki parameters κ 1 and κ 2 is much higher than expected from any present theory.

Critical data of niobium nitride in transverse magnetic fields

Critical fields and critical currents of a great number of niobium nitride samples with and without different amounts of zirconium have been measured. The samples for these measurements have been prepared by diffusion of nitrogen into heated niobium and niobium-zirconium wires under various conditions. For the zero upper critical field of NbN the extrapolated value (153±3) kOe has been found. The quenching curves of most of the samples show very pronounced peak effects in the high field region with a steep slope close to the upper critical field, which decreases with increasing amount of zirconium.

Low temperature specific heats of unannealed niobium wires in magnetic fields

Specific heats of a bundle of unannealed niobium wires have been measured at different magnetic fields. Emphasis is given to the mixed superconductive state at temperatures much below the transition temperature T c, as well as to the temperature dependence of the specific heat jump at the transition from the mixed to the normal state.

Untersuchungen im System Niob-Stickstoff: III. Kinetik der Entgasung von Niob-Stickstoff-Mischkristallen im Hochvakuum

The degassing kinetics of niobium-nitrogen solid solutions were determined by annealing in high vacuum between 1630 and 1970° C. Nitrogenated niobium wires of 0.5 mm dia, containing about 3 at % N, were resistance-heated by dc. Continuous resistance measurements during annealing served to follow the degassing process. In order to interpret the degassing experiments, the rate and degassing laws and also the expressions for the rate constants and half-times were computed for various rate-determining sub-processes. Detectable degassing begins above 1600° C; above 2000° C a few minutes sufficed to achieve complete degassing. The degassing can be described by a rate law of second order with an activation energy of 124 kcal/mole N 2. The rate-determining factor is taken to be the recombination of nitrogen atoms at the metal surface and their subsequent desorption. A relationship is quoted for the temperature dependence of the degassing rate which is valid for nitrogenated niobium samples which are thin but otherwise of arbitrary shape.

Superconductivity of a Composite of Fine Niobium Wires in Copper

Composites consisting of fine Nb filaments in a copper matrix were fabricated, and the following superconducting properties measured: the critical temperature, critical transverse magnetic field as a function of temperature, critical current density at 4.2°K as a function of transverse magnetic field, and magnetization at 4.2°K. The reduction in Nb filament size was checked by optical and electron micrography. Filament diameters ranged down to 100 Å. As the diameter decreased, Tc, Hc(T), and Jc all decreased. This behavior was attributed to the proximity effects of the copper matrix on the Nb. The results are closely comparable to those of investigations of superimposed thin films. It is therefore not expected that high critical fields can be attained by composites involving a type II superconductor in a matrix of a nonsuperconductor.

Der Einflu� von Strom und Magnetfeld auf die Supraleitungseigenschaften von Nb3Sn-, V3Ga- und V3Si-Diffusionsschichten

For diffusion layers of Nb3Sn, V3Ga, and V3Si on niobium and vanadium wires, respectively, the transition to superconductivity has been measured at various currents in longitudinal and transverse magnetic fields. Current and field shift the transition curve to lower temperatures, except near the onset of superconductivity where the core of the wire influences the transition. The critical current vs. temperature curves are linear at higher currents, the critical field curves are bended. The results are explained by local variations of the transition temperature due to inhomogeneities and by a displacement of the current paths under the influence of the Lorentz force. For very thin diffusion layers an influence of thickness on the transition temperature has been observed. In addition quenching curves of the diffusion layers have been measured. The shape is in accordance with the model ofKim and coworkers. The absolute values and deviations at higher fields are discussed. A deduction of the critical values at low temperatures from measurements near the transition is possible.

Stress relaxation and thermal activation in niobium

Values of activation volume and energy have been determined for yielding, flow and stress relaxation in high purity polycrystalline niobium wire containing less than 100 ppm of interstitial impurities. It was found that within the scatter of the experimental results the values of activation volume as a function of the effective stress were the same for yielding, flow and relaxation, suggesting that the same dislocation mechanism was rate controlling in all three cases. It was tentatively suggested that the dislocation mechanism that was rate controlling was either that of overcoming Peierls-Nabarro stresses, or that of the conservative motion of jogs. A new relationship was developed for the analysis of the stress relaxation data to account for relaxation in the tensile testing machine.

Oscillatory longitudinal paramagnetic effect in a superconducting niobium wire carrying current

Oscillatory longitudinal paramagnetic effect in a superconducting niobium wire carrying current

Electrical Resistance Measurements on Hydrogen‐Charged Tantalum and Niobium

AbstractBy measuring the electrical resistance of tantalum and niobium wires, it is possible to detect the effective introduction of hydrogen in these b.c.c. metals by electrolytic charging. In both metals this process gives an increase of the electrical resistivity which, for low concentrations, at room temperature is ΔϱH = 0.80 μΩ cm/at.‐% hydrogen in tantalum and ΔϱH = 0.75 μΩ cm/at.‐% of hydrogen in niobium.Subsequent isochromal annealings in vacuum (10−4 mm Hg) shows that the hydrogen introduced by electrolytic charging is completely removed at temperatures above 800 °C. This degasing process becomes important only for temperatures above 500 °C for tantalum and 400 °C for niobium.

X-Ray Study of Wire-Drawn Niobium and Tantalum

AbstractDeformation, introduced into niobium and tantalum specimens by wire drawing at room temperature, produced changes in the shape and position of X-ray diffraction peaks. The resultant peak profiles and locations of all available peaks were recorded using the Debye-Scherrer geometry on a modified dtffractometer with crystal monochromated Cu Kα radiation. The amount of deformation in the surface layers of both metals was found to saturate essentially after only 20% reduction in area. The measured decrease in the lattice parameters of either material was attributed to a residual surface stress; the average value for the deformed saturated state for both tantalum and niobium wires corresponded to an equivalent longitudinal tensile stress of 35 ± 5 kg/mm2. Integral breadth measurements revealed approximately equal X-ray particle sizes in the <100> and <110> directions; the minimum particle size for the micro structures of both metals was around 200 Å and occurred after the first few draws.

A NEW METHOD FOR MEASURING THE CRITICAL CHARACTERISTIC CURVES OF SUPERCONDUCTORS

In this paper, a persistent-current method for measuring critical characteristic curves of superconductors has been described. In comparison with the usual four-probe method, this new method possesses the following advantages: (1) The lc (H) curves thus determined have clear physical significance. (2) The technical difficulty of introducing heavy current into the cryostat is avoided. (3) There is no danger of burn-out of the specimen tested. The Ic(H) curve of cold worked niobium wire has been determined with this method. The result confirms the existence of the peak in the lc (H) characteristic observed previously with the four-probe method.